The present invention relates to a current-powered converter comprising a direct-current source which powers a switching section connected to a transformer which transfers power to a load via the output of the converter, or more generally a switching section connected to the output of the converter via a circuitry which has a dispersed inductance.
In current-powered converters, during the switching cycle of the switches in the switching section, situations occur where the voltage at the input of the switching section increases owing to the dispersed inductance of the transformer provided between the switching section and the converter output. Similar situations may also occur in the presence of dispersed inductances due to other circuit components and therefore in the absence of the output transformer. In order to avoid the occurrence of excessive voltages at the terminals of the electronic switches of the switching section, voltage limiting circuitsxe2x80x94typically called clamping circuitsxe2x80x94are used.
An object of the present invention is to provide a clamping circuit which is particularly simple and which allows recovery of the energy stored during the operative phase of the circuit such that it can be returned during a suitable phase of the switching cycle of the switches of the switching section.
Essentially, the clamping circuit according to the invention comprises a capacitor, in which energy is stored during at least one phase of the switching cycle of the converter, and an inductance, by means of which the energy stored in the capacitor is returned to the output of the converter.
Basically, according to one embodiment of the invention, a unidirectional component, typically a diode, is arranged in series with the capacitor. The diode and the capacitor in series are arranged between the positive terminal and the negative terminal on the input side of the switching section. Moreover, a branch comprising the inductance and a controlled switch, which is closed in order to transfer energy to the load, is arranged in parallel with the diode. Switching of the switch is controlled so that it is closed when the switching section is in a phase where it transfers energy to the output of the converter, i.e. to the load, and when the voltage at the terminals of the capacitor is equal to at least a reference voltage.
When the controlled switch is closed, the capacitor is electrically connected to the inductance so as to allow transfer, to the inductance, of the energy stored during the previous phase in the electrical field of the capacitor. The energy is then transferred from the inductance across the switching section and therefore toward the load.
According to another embodiment of the invention, a second unidirectional re-circulating component, typically a diode, is arranged in parallel with the controlled switch and the capacitor of the clamping circuit, said component preventing the occurrence of over-voltages when the controlled switch is opened.
Switching of the controlled switch is triggered by a switching circuit which prevents closing of the controlled switch until the voltage at the terminals of the capacitor of the clamping circuit has reached a predetermined value. When this value has been reached, the switching circuit causes the controlled closing and opening of the switch depending on the switching conditions of the switching section. In this way, it is ensured that the clamping circuit enters into operation only when a relatively high voltage is present at the terminals of the capacitor. This ensures the possibility of recovery of the energy by causing a relatively low current to flow through the inductance, thereby limiting the losses. Moreover, keeping the current values across the inductance low allows the use of an inductance which has small dimensions and therefore is not very bulky and is low-cost, resulting in advantages in terms of the size and cost of the entire circuit.
Further advantageous features and embodiments of the converter according to the invention are indicated in the accompanying drawings and will be described with reference to a non-limiting example of embodiment of the invention.